Enrollment in the new health care reform law’s private insurance exchanges continues to lag among younger Americans, according to figures released by the Obama administration Wednesday.

About 3.3 million Americans enrolled in an insurance plan through the state or federal marketplaces created by the Affordable Care Act between Oct. 1 and the end of January, according to the Department of Health and Human Services. But only 25 percent of those enrolled were between the ages of 18-34, well below the 40 percent benchmark the administration set for the age group. Younger, healthier enrollees are necessary to subsidize coverage for older enrollees.

“It’s very, very encouraging news,” Health and Human Service Secretary Kathleen Sebelius, speaking on a conference call with reporters, said of the uptick in total enrollment.

The four-month enrollment total is still only the equivalent of what the administration had hoped to have sign-on for coverage just by the end of 2013, before massive technical problems with the HealthCare.gov website marred its launch in October.

The percentage of 18-34 year-olds who enrolled was slightly better for the month of January than the three prior months, coming in at 27 percent compared to 24 percent. Officials predict that so-called “young invincibles” will enroll later in the six-month process, and are stepping up their outreach efforts to bring them aboard.

The Obama administration has not released data on what percentage of who have enrolled in plans have in fact paid premiums to begin receiving coverage.

Republicans seized on the lag in younger enrollment to again paint the law as a failure.

“With embarrassing failures, broken promises, shifting deadlines, and miserable enrollment numbers, the last few months have been a rolling catastrophe,” said Brendan Buck, a spokesman for House Speaker John Boehner. “Under penalty of higher taxes, Americans are mandated by law to have a plan, but they’re still resisting Obamacare. And many that are actually signing up are only doing so because the law cancelled the plan they had, liked, and could afford. This is what happens when you offer one-size-fits-all plans that come with high costs for limited access to your doctor.”